Dye lasers (lasers wherein a dye is used as a gain-medium) can be tuned over a relatively wide spectral range. Any one such dye can provide a tuning range of about 50 nanometers (nm). The range of such dyes available provides that these tuning ranges are available in a total spectral range ranging from about 320 nm to about 1000 nm. Because of this, dye lasers continue to be favored for research and spectroscopic applications of lasers.
A laser dye has a broad gain-bandwidth. This would cause laser radiation emitted by a dye laser to have a relatively broad bandwidth, absent any provision in the laser resonator to restrict the emitted bandwidth. Ideally the emitted bandwidth of the laser should be no wider than spectral absorption lines in materials being investigated with, or processed by, the laser radiation. Providing such line restriction in a laser resonator is termed “line-narrowing” by practitioners of the laser art.
In an extensively used resonator configuration for providing line narrowing, the resonator is formed between a reflective diffraction grating and a conventional mirror. The mirror may be partially transmitting for coupling radiation out of the resonator. Alternatively, an optical device such as a grazing incidence prism may be used to couple radiation out of the resonator. Radiation generated in such a resonator is within a diffraction order of the grating. The angle of the radiation diffracted from the grating determines the center wavelength of the laser radiation and varying this angle can provide tuning of the laser within the tuning range (gain-bandwidth) of a particular dye. The dispersion provided by the grating limits the range of wavelengths around a particular center wavelength that can circulate in the resonator, thereby providing the line narrowing.
In certain applications, line narrowing provided by the diffraction grating alone may not be sufficient. Further line narrowing for such applications is usually accomplished by placing an etalon within the diffraction-grating-terminated laser resonator. One such resonator configuration is schematically illustrated in FIG. 1, here, a dye laser 10 includes a resonator 12 terminated by a mirror 14 and a diffraction grating 16 having rulings 16R. Resonator 12 includes a dye cell 17 that contains the gain-medium (dye) for the laser. The gain-medium is energized by light from an externally frequency multiplied, neodymium-doped YAG (Nd:YAG) laser 20. Laser 20 in this example delivers frequency-doubled light having a wavelength of about 530 nanometers (nm) via an aperture 22. Alternatively the laser can deliver frequency-tripled light having a wavelength of about 353 nanometers (nm) via an aperture 24. Paths of light from aperture 22 are indicated by dotted lines with the direction of propagation of the light being indicated by open arrowheads.
Light from aperture 22 is directed by a mirror 26 to a beamsplitter 28. Dotted outline 26A indicates an alternative position for mirror 26 for a case where the frequency-tripled light from aperture 24 is used for energizing the dye cell. Beamsplitter 28 reflects a first portion of the light toward dye cell 18 and transmits a second portion of the light. The reflected portion of the light is focused by a spherical lens 30 and a cylindrical lens 32 into the dye cell causing a beam of laser radiation to circulate in resonator 12. Cylindrical lens 32 is oriented such that the light is focused along the path of the beam of laser radiation circulating in resonator 12, here, indicated by solid line F.
A portion FOUT of the radiation circulating in then resonator is reflected at grazing incidence from surface 19 of a prism 18 included in the resonator. Radiation FOUT is directed by a mirror 34 back through dye cell 17. The portion of pump light transmitted by beamsplitter 28 is directed by mirrors 36 and 38 to a beamsplitter 40. Beamsplitter 40 reflects a portion of the light incident thereon and transmits a further portion of that light. The reflected portion of the light is focused by a spherical lens 42 and a cylindrical lens 44 into the path of radiation FOUT traversing the dye cell. Accordingly radiation FOUT is amplified by passage thereof through the dye-cell. The amplified radiation, here, is spatially filtered by a pinhole aperture 46. After spatial filtering the light is relayed by lenses 48 and 50 to another dye-cell amplifying stage (not shown). The dye cell is pumped (energized) by the portion of light transmitted by beamsplitter 40.
Radiation F entering transmitted through surface 19 of prism 18 exits the prism via surface 21 thereof. The transmitted radiation is expanded, in one axis only, by a beam expanding prism 34, thereby providing an expanded beam having an elongated cross-section. Limits of the expanded beam are indicated by solid lines FS and FL. The expanded beam passes through an etalon 52 onto diffraction grating 16. The expanded beam is incident on the diffraction grating generally at an angle corresponding to that of the diffracted order for a center wavelength around which the output spectrum of the laser is to be narrowed. The center wavelength can be tuned by rotating or tilting diffraction grating 16 about an axis 15 as indicated by double arrow T. Wavelengths close to the center wavelength will return from the diffraction grating through etalon 52 along or close to the paths followed to be incident on the grating. Beam expanding prism 34, of course, compresses the expanded beam returning along the incidence path. One disadvantage of this line narrowing system is that during tuning of the center wavelength the etalon must be rotated synchronously with the grating to match the peak transmission wavelength of the etalon to center wavelength determined by rotating the diffraction grating.